The present invention relates to a luminescence crystal particle which emits light upon irradiation with an energy beam, a luminescence crystal particle composition, a display panel constituted of such luminescence crystal particles and a flat-panel display having such a display panel.
As an image display device that can be substituted for a currently mainstream cathode ray tube (CRT), flat-screen (flat-panel) displays are studied in various ways. Such fat-panel displays include a liquid crystal display (LCD), an electroluminescence display (ELD) and a plasma display (PDP). There has been also proposed a cold cathode field emission display capable of emitting electrons into a vacuum from a solid without relying on thermal excitation, a so-called field emission display (FED), and it attracts attention from the viewpoint of the brightness of a display screen and low power consumption.
FIG. 1 shows a typical constitution of the cold cathode field emission display. In this display, a display panel 20 and a back panel 10 are placed so as to face each other, and these two panels 10 and 20 are bonded to each other through a frame (not shown) in their circumferential portions. A space closed with these two panels forms a vacuum space. The back panel 10 has cold cathode field emission devices (to be referred to as xe2x80x9cfield emission devicesxe2x80x9d hereinafter) as electron-emitting elements. One example shown in FIG. 1 is a so-called Spindt-type field emission device having a conical electron-emitting portion 16. The Spindt-type field emission device comprises a stripe-shaped cathode electrode 12 formed on a substrate 11; an insulating layer 13 formed on the cathode electrode 12 and the substrate 11; a stripe-shaped gate electrode 14 formed on the insulating layer 13; and a conical electron-emitting portion 16 formed in an opening portion 15 formed in the gate electrode 14 and the insulating layer 13. The electron-emitting portion 16 is formed on a portion of the cathode electrode 12 which portion is positioned in a bottom portion of the opening portion 15. Generally, a number of such electron-emitting portions 16 are formed to correspond to one of phosphor layers 22 to be described later. A relatively negative voltage (video signal) is applied to the electron-emitting portion 16 from a cathode electrode driving circuit 31 through the cathode electrode 12, and a negatively positive voltage (scanning signal) is applied to the gate electrode 14 from a gate electrode driving circuit 32. An electric field is generated due to the application of these voltages, and due to the electric field, electrons are emitted from the top end of the electron-emitting portion 16 on the basis of a quantum tunnel effect. The electron-emitting element shall not be limited to the above Spindt-type field emission device, and field emission devices of other types such as edge-type, flat-type or crown-type field emission devices are used in some cases. Further, reversibly, the scanning signal may be inputted to the cathode electrode 12, and the video signal may be inputted to the gate electrode 14.
The display panel 20 has a plurality of phosphor layers 22 which are formed on a support member 21 made of glass or the like and have the form of dots or stripes, and an anode electrode 24 made of an electrically conductive reflection film formed on the phosphor layers 22 and the support member 21. A positive voltage higher than the positive voltage applied to the gate electrode 14 is applied to the anode electrode 24 from an accelerating power source (anode electrode driving circuit) 33, and it works to guide electrons emitted from the electron-emitting portion 16 to the vacuum space toward the phosphor layer 22. Further, the anode electrode 24 functions to protect the phosphor particles constituting the phosphor layer 22 from sputtering by particles such as ions, functions to reflect light emitted from the phosphor layers 22 on the basis of electron excitation to the side of the support member 21 to improve the brightness of a display screen observed from an outside of the support member 21, and functions to prevent excess charge to stabilize the potential of the display panel 20. That is, the anode electrode 24 not only carries out its function as an anode electrode but also carries out the function of a member known as a metal back layer in the field of a cathode ray tube (CRT). The anode electrode 24 is generally constituted of a thin aluminum film. A black matrix 23 is formed between one phosphor layer 22 and another phosphor layer 22.
FIG. 2A shows a schematic plan view of the display panel having phosphor layers 22R, 22G and 22B formed in the form of dots, and FIG. 2B shows a schematic partial cross-sectional view taken along a line Xxe2x80x94X in FIG. 2A. A region where the phosphor layers 22R, 22G and 22B are arranged is an effective field which carries out a practical function, and a region where the anode electrode is formed is nearly in agreement with the effective field. For clear showing in FIG. 2A, the region where the anode electrode is formed is provided with slanting lines. A circumferential region to the effective field is an ineffective field for supporting the function of the effective field, where peripheral circuits are formed and a display screen is mechanically supported.
In the cold cathode field emission display, the anode electrode is not necessarily required to be constituted of the anode electrode 24 made of an electrically conductive reflection film. It may be constituted of an anode electrode 25 made of a transparent electrically conductive film formed on the support member 21, as is shown in FIG. 2C which is a schematic partial cross-sectional view similarly taken along a line Xxe2x80x94X in FIG. 2A. On the support member 21, each of the anode electrodes 24 and 25 is formed nearly on the entire surface of the effective field.
FIG. 3A shows a schematic plan view of the display panel having the phosphor layers 22R, 22G and 22B formed in the form of stripes, and FIGS. 3B and 3C show schematic partial cross-sectional views taken along a line Xxe2x80x94X in FIG. 3A. In FIGS. 3A, 3B and 3C, the same portions as those in FIGS. 2A, 2B and 2C are shown by the same reference numerals, and detailed explanations of the same portions are omitted. FIG. 3B shows a constitution in which the anode electrode 24 is made of an electrically conductive reflection film, and FIG. 3C shows a constitution in which the anode electrode 25 is made of a transparent electrically conductive film. Each of the anode electrodes 24 and 25 is formed nearly on the entire surface of the effective field of the display panel.
In the cold cathode field emission display that is a flat-panel display, the flying distance of electrons is far smaller than the counterpart in a cathode ray tube, so that it is difficult to increase an electron-accelerating voltage to the level of an electron-accelerating voltage in the cathode ray tube. In the cold cathode field emission display, if the electron-accelerating voltage is too high, spark discharge is liable to take place between the electron-emitting portion in the back panel and the film which functions as an anode electrode in the display panel, and the display quality of the cold cathode field emission display may be impaired to a great extent. The accelerating voltage is therefore controlled to be approximately 10 kilovolts or lower.
In addition to the above problem, the cold cathode field emission display for which it is required to select the above low electron-accelerating voltage involves characteristic problems from which the cathode ray tube is free. In a cathode ray tube permitting the acceleration at a high voltage, electrons enter the phosphor layers deep, so that the electron energy is received in a relatively broad region inside the phosphor layers to excite a relatively large number of phosphor particles present in such a broad region at once, and high brightness can be attained. When the accelerating voltage is set at 31.5 kilovolts and when the phosphor layer is made of ZnS, Monte Carlo simulation is conducted with regard to a relationship between an energy loss of electrons which have entered the phosphor layer and the electron penetration depth into the phosphor layer on the basis of the Bethe expression represented by the following equation (1) (see xe2x80x9cpractical Scanning Electron Microscopyxe2x80x9d, J. I. Goldstein and H. Yokowitz, p 50, Plenun Press, New York (1975)). FIG. 32 shows the result thereof. It is seen from FIG. 32 that when the accelerating voltage is 31.5 kilovolts, the peak of electron energy loss is positioned approximately 1 xcexcm apart from the surface of the phosphor layer. Further, electrons enter approximately 5 xcexcm deep from the surface of the phosphor layer.
xe2x88x92(dEm/dX)=2xcfx80e4N0(Z/A)(xcfx81/Em)1n(1.166Em/J)xe2x80x83xe2x80x83(1)
In the cold cathode field emission display, however, the accelerating voltage is required to be approximately 10 kilovolts or lower, for example, approximately 6 kilovolts. When the accelerating voltage is set at 6 kilovolts and when the phosphor layer is made of ZnS, Monte Carlo simulation is conducted with regard to a relationship between an energy loss of electrons which have entered the phosphor layer and the electron penetration depth into the phosphor layer on the basis of the above Bethe expression, and FIGS. 33 and 34 show the results. In FIG. 33, a 0.045 xcexcm thick aluminum thin film is formed on the surface of the phosphor layer, and in FIG. 34, a 0.07 xcexcm thick aluminum thin film is formed on the surface of the phosphor layer. FIGS. 33 and 34 show that the peak of electron energy loss is positioned near the outermost surface of the phosphor layer. Further, electrons enter only approximately 0.2 to 0.3 xcexcm deep from the surface of the phosphor layer. In the cold cathode field emission display in which the accelerating voltage is lower than that in the cathode ray tube, the electron penetration depth into the phosphor layer is small, and the electron energy is received only in a narrow region of the phosphor layer (particularly, only near the surface of the phosphor layer).
In the phosphor layer, further, 10% of the energy of electrons contributes to light emission, and the remaining approximately 90% of the energy is converted to heat. That is, heat is generated greatly near the surface of the phosphor layer. As a result, when the phosphor layer is constituted of phosphor particles made of a sulfide, sulfur that is a component therefore is dissociated in the form of a single atom or in the form of sulfur monoxide (SO) or sulfur dioxide (SO2), and the phosphor particles made of a sulfide alter in composition or a luminescence center disappears. When the accelerating voltage is set at 6 kilovolts and when the phosphor layer is made of ZnS, Monte Carlo simulation is conducted with regard to a relationship between an energy loss of electrons which have entered the phosphor layer and the electron penetration depth into the phosphor layer on the basis of the above Bethe expression, and FIG. 35 shows the result thereof. In FIG. 35, it is assumed that a 0.07 xcexcm thick aluminum thin film is formed on the surface of the phosphor layer and that Zn is formed due to dissociation of sulfur (S) from ZnS in a thickness ranging from the surface of the phosphor layer to a portion approximately 0.03 xcexcm deep from the surface. FIG. 35 clearly shows that the peak of electron energy loss is positioned in a region of the phosphor layer which region is made of Zn due to the dissociation of sulfur (S) from ZnS. Further, electrons reaches only approximately 0.2 xcexcm deep from the surface of the phosphor layer.
In the cold cathode field emission display, further, the position in the phosphor layer (more specifically, phosphor particles) with which position electrons emitted from one field emission device collide is generally constant unlike the cathode ray tube. Therefore, the phosphor particles with which the electrons collide constantly is deteriorated greatly as compared with other phosphor particles, and the phosphor particles are deteriorated faster that the counterpart in the cathode ray tube.
Further, the outermost surface of the phosphor particle suffers various strains during the processes of producing the phosphor particles and producing the display panel and is liable to have lattice defects. Moreover, it is required to drive the cold cathode field emission display at a higher current density (emitted-electron density) than the cathode ray tube for attaining desired brightness. For example, a current density in the cathode ray tube is 0.1 to 1 xcexcA/cm2, while the cold cathode field emission display requires a current density of as high as 5 to 10 xcexcA/cm2. It is therefore required to operate the outermost surface of the phosphor particle or a portion nearby under high-excitation conditions. While the cold cathode field emission display is operated, crystal defects are liable to be formed or multiplied newly in the phosphor particle, which is considered to cause the brightness deterioration to proceed faster.
The above-explained deterioration of the phosphor layer or the phosphor particles results in the fluctuation of emitted-light color and luminescence efficiency, the contamination of internal components of the cold cathode field emission display and a consequent decrease in reliability and lifetime characteristics of the cold cathode field emission display. It is therefore strongly desired to develop a phosphor layer or phosphor particles free from deterioration for improving the cold cathode field emission display in reliability and lifetime characteristics.
For attaining finer display with a cathode ray tube, it is required to decrease a diameter of an electron beam that collides with the phosphor layer. That is, it is required to increase the current density of the electron beam that collides with the phosphor layer. In this method, however, the phosphor particles that emit light in green are particularly liable to be damaged, and such a phenomenon leads to the generation of a magenta ring. The above magenta ring refers to a phenomenon in which the phosphor particles that emit light in red and light in blue are scarcely damaged, and in the cathode ray tube, a magenta color that is a complementary color to green is observed in the form of a ring. In the conventional cathode ray tube, the current density of the electron beam that collides with the phosphor layer and the lifetime of the cathode ray tube are inversely proportional to each other. For preventing a decrease in the lifetime of the cathode ray tube while increasing the current density of the electron beam that collides with the phosphor layer, it is strongly desired to develop a phosphor layer or phosphor particles which is/are deteriorated to a less degree.
It is therefore an object of the present invention to provide luminescence crystal particles that are deteriorated to a less degree even in use for a long period of time, namely, that suffer a decrease in brightness to a less degree, a display panel constituted of such luminescence crystal particles, a flat-panel display having such a display panel, and a luminescence crystal particle composition.
According to the present invention, the above object can be achieved by a luminescence crystal particle which emits light upon irradiation with an energy beam and which has a crystal defect density of 5xc3x97107 defects/cm2 or less in a region located from the surface of the luminescence crystal particle to a portion as deep as the energy beam reaches.
The luminescence crystal particle of the present invention or a luminescence crystal particle composition to be described later can be used for constituting, for example, a cold cathode field emission display or the front panel (anode panel) thereof; a commercial (home-use), industrial (for example, a computer display), digital broadcasting or a projection type cathode ray tube or a face plate thereof; or a plasma display or a rear panel thereof. The rear panel for an AC driven or DC driven plasma display comprises, for example, a support member; separation walls (ribs) formed on the support member; various electrodes (for example, data electrode) formed on the support member located between one separation wall and another separation wall; and a luminescence layer made of the luminescence crystal particles formed between one separation wall and another separation wall. The front panel (anode panel) of the cold cathode field emission display and the face plate of the cathode ray tube will be discussed later.
According to the present invention, the above object can be also achieved by a display panel comprising a support member, a luminescence layer made of luminescence crystal particles which emit light upon irradiation with electrons flying from a vacuum space, and an electrode,
said luminescence crystal particle having a crystal defect density of 5xc3x97107 defects/cm2 or less in a region located from the surface of the luminescence crystal particle to a portion as deep as the electrons reach.
The display of the present invention includes a so-called face plate of a commercial (home-use), industrial (computer display), digital broadcasting or projection type cathode ray tube; or a front panel (anode panel) for a cold cathode field emission display. The face plate for a cathode ray tube generally comprises a glass panel (corresponding to the support member of the display panel of the present invention); luminescence layers made of the luminescence crystal particles and formed on an inner surface of the glass panel in the form of stripes or dots; a black matrix formed on the inner surface of the glass panel between one luminescence layer and another luminescence layer; and a metal back layer (corresponding to the electrode of the display panel of the present invention) formed on the luminescence layers and the black matrix. The front panel (anode panel) of a cold cathode field emission display comprises a support member; luminescence layers made of the luminescence crystal particles and formed in the form of stripes or dots (luminescence layers which are patterned in the form of stripes or dots and correspond to three primary colors, red (R), green (G) and blue (B), and are alternately arranged for a color display); and an anode electrode (corresponding to the electrode of the display panel of the present invention). A black matrix may be formed between one luminescence layer and another luminescence layer.
Further, according to the present invention, the above object can be achieved by a flat-panel display having a display panel and a back panel having a plurality of electron-emitting regions, wherein the display and the back panel face each other through a vacuum space,
said display panel comprising a support member, a luminescence layer made of luminescence crystal particles which emit light upon irradiation with electrons flying from the electron-emitting region, and an electrode, and
said luminescence crystal particle having a crystal defect density of 5xc3x97107 defects/cm2 or less in a region located from the surface of the luminescence crystal particle to a portion as deep as the electrons reach.
The display panel of the flat-panel display of the present invention includes the front panel (anode panel) of the above cold cathode field emission display.
In the display panel of the present invention or the display panel of the flat-panel display of the present invention, the luminescence layer can be formed by a screen printing method or a slurry method. In the screen printing method, the luminescence crystal particle composition of the present invention to be described later is printed on the support member (on the electrode and the support member in some cases), the applied composition is dried and calcined, whereby the luminescence layer can be formed. In the slurry method, the luminescence crystal particle composition of the present invention containing a photosensitive polymer and being in the state of a slurry is applied to the support member (to the electrode and the support member in some cases), and then, the photosensitive polymer is insolubilized to a developer solution by exposure to light, whereby the luminescence layer can be formed. For displaying three primary colors of (R,G,B), three luminescence crystal particle compositions or three slurries are consecutively used, and the luminescence layers for emitting light in such three colors can be formed by the screen printing method or the slurry method.
According to the present invention, the above object can be achieved by a luminescence crystal particle composition comprising a dispersion of luminescence crystal particles which emit light upon irradiation with an energy beam and each of which has a crystal defect density of 5xc3x97107 defects/cm2 or less in a region located from the surface of the luminescence crystal particle to a portion as deep as the energy beam reaches, in a dispersing medium.
In the luminescence crystal particle composition of the present invention, water can be used as a dispersing medium. The luminescence crystal particle composition of the present invention may contain polyvinyl alcohol as a dispersing agent or a retaining agent, and ammonium bichromate may be used as a photosensitive polymer. The luminescence crystal particles of the present invention may be surface-treated for improving the dispersing property and adhesion thereof.
In the luminescence crystal particle, the display panel, the flat-panel display or the luminescence crystal particle composition (these will be sometimes referred to as xe2x80x9cthe present inventionxe2x80x9d hereinafter), more preferably, the crystal defect density in a region located from the surface of the luminescence crystal particle to a portion as deep as the energy beam reaches or the electrons reach is 1xc3x97107 defects/cm2 or less. The above crystal defect includes dislocation, stacking fault and twin boundary.
The energy beam or electron penetration depth from the surface of each luminescence crystal particle can be determined by assuming the energy of an energy beam or electrons and a material constituting the luminescence crystal particles and carrying out Monte Carlo simulation of the energy loss of the energy beam or the electrons which has/have entered each luminescence crystal particle and the energy beam or electron penetration depth in the luminescence crystal particle on the basis of the above Bethe expression. In the simulation, it is assumed that the electrons lose their energy of an average of approximately 43 eV (mean free path of approximately 4.8 nm) per scattering and halt after they undergo elastic scattering 150 times.
Further, the crystal defect density of crystal defects such as dislocations, stacking faults and twin boundaries can be determined by observing the luminescence crystal particle(s) through a transmission electron microscope, counting crystal defects in an area having a size of 5 xcexcmxc3x975 xcexcm in the luminescence crystal particle and converting the counted number to a density per cm2.
In the present invention, the luminescence crystal particle includes a phosphor particle. The blue-emitting phosphor particle includes ZnS:Ag, ZnS:Ag,Al and ZnZ:Ag,Cl. The green-emitting phosphor particle includes Zn2SiO4:Mn2+, (Zn,Cd)S:Ag, (Zn,Cd)S:Cu and ZnS:Cu,Al. The red-emitting phosphor particle includes Zn3(PO4)2:Mn2+, (Zn,Cd)S:Ag, YVO4:Eu3+, Y2O2S:Eu3+ and Y2O3:Eu3+. Further, the reddish orange-emitting phosphor particle includes Y2O2S:Eu3+, and the marine blue-emitting phosphor particle includes ZnS:Ag.
In the luminescence crystal particle or the luminescence crystal particle composition of the present invention, an electron beam can be used as an energy beam. The energy of the electron beam for irradiation of the luminescence crystal particle is preferably set at 0.5 keV to 35 keV. In the above constitution, specifically, the luminescence crystal particle can be used for constituting a cold cathode field emission display or a front panel (anode panel) thereof, a commercial (home-use), industrial (for example, computer display), digital broadcasting or projection type cathode ray tube or a face plate thereof. Otherwise, there may be employed a constitution in which the energy of the electron beam for irradiation of the luminescence crystal particle is 0.5 keV to 10 keV and the electron penetration depth from the surface of the luminescence crystal particle is 0.5 xcexcm or less. In the above constitution, specifically, the luminescence crystal particle can be used for constituting a cold cathode field emission display or a front panel (anode panel) therefore. Otherwise, in the luminescence crystal particle of the present invention, an ultraviolet ray can be used as an energy beam. In this case, preferably, the ultraviolet ray for irradiation of the luminescence crystal particle has a wavelength of 100 nm to 400 nm. In the above constitution, specifically, the luminescence crystal particle can be used for constituting a plasma display or a rear panel therefore.
In the present invention, desirably, the average particle diameter of the luminescence crystal particles is 1xc3x9710xe2x88x928 m to 1xc3x9710xe2x88x925 m, preferably 1xc3x9710xe2x88x926 m to 1xc3x9710xe2x88x925 m, more preferably 4xc3x9710xe2x88x926 m to 8xc3x9710xe2x88x926 m. The luminescence crystal particles can be measured for an average particle diameter D50 according to a light scattering method or a Coulter counter method. Preferably, the luminescence crystal particle has an average surface roughness of 5 nm or less. The average surface roughness is defined to be that which is obtained by observing the luminescence crystal particle through a transmission electron microscope, measuring concave and convex portions of the luminescence crystal particle, determining level differences between bottom positions of the convex portions where the surface changes from the concave portions to the convex portions and peak positions where the surface changes from convex portions to the concave portions and further determining an average of the level differences as an average surface roughness.
The flat-panel display of the present invention can have a structure in which each electron-emitting region comprises one cold cathode field emission device or a plurality of cold cathode field emission devices,
said cold cathode field emission device comprises;
(A) a substrate,
(B) a stripe-shaped cathode electrode formed on the substrate,
(C) an insulating layer formed on the substrate and the cathode electrode,
(D) a stripe-shaped gate electrode formed on the insulating layer,
(E) an opening portion penetrating through the gate electrode and the insulating layer, and
(F) an electron-emitting portion formed on a portion of the cathode electrode which portion is positioned in the bottom portion of the opening portion, and
the electron-emitting portion exposed in the bottom portion of the opening portion is for emitting electrons.
The above structure will be referred to a cold cathode field emission device having a first structure. The type of the above cold cathode field emission device includes a Spindt-type cold cathode field emission device (cold cathode field emission device having a conical electron-emitting portion formed on a portion of the cathode electrode which portion is positioned in the bottom portion of the opening portion), a crown-type cold cathode field emission device (cold cathode field emission device having a crown-shaped electron-emitting portion formed on a portion of the cathode electrode which portion is positioned in the bottom portion of the opening portion), and a plane-type cold cathode field emission device (cold cathode field emission device having a nearly flat electron-emitting portion formed on a portion of the cathode electrode which portion is positioned in the bottom portion of the opening portion).
Otherwise, the flat-panel display of the present invention can have a structure in which each electron-emitting region comprises one cold cathode field emission device or a plurality of cold cathode field emission devices,
said cold cathode field emission device comprises;
(A) a substrate,
(B) a stripe-shaped cathode electrode formed on the substrate,
(C) an insulating layer formed on the substrate and the cathode electrode,
(D) a stripe-shaped gate electrode formed on the insulating layer, and
(E) an opening portion penetrating through the gate electrode and the insulating layer, the cathode electrode being exposed in the bottom portion of the opening portion, and
the portion of the cathode electrode which portion is exposed in the bottom portion of the opening portion is for emitting electrons.
The above structure will be referred to as a cold cathode field emission device having a second structure. The type of the above cold cathode field emission device includes a flat-type cold cathode field emission device which emits electrons from the flat surface of the cathode electrode and a crater-type cold cathode field emission device which emits electrons from a convex portion of the surface of the cathode electrode having convexo-concave form.
Further, the flat-panel display of the present invention can have a structure in which each electron-emitting region comprises one cold cathode field emission device or a plurality of cold cathode field emission devices,
said cold cathode field emission device comprises;
(A) a substrate,
(B) a stripe-shaped cathode electrode having an edge portion formed on or above the substrate,
(C) an insulating layer formed at least on the cathode electrode,
(D) a stripe-shaped gate electrode formed on the insulating layer, and
(E) an opening portion penetrating through at least the gate electrode and the insulating layer, and
the edge portion of the cathode electrode which edge portion is exposed in the bottom portion or the side wall of the opening portion is for emitting electrons.
The above structure will be referred to as a cold cathode field emission device having a third structure or an edge-type cold cathode field emission device.
In the cold cathode field emission device having the first, second or third structure, the material for constituting the gate electrode includes at least one metal selected from the group consisting of tungsten (W), niobium (Nb), tantalum (Ta), titanium (Ti), molybdenum (Mo), chromium (Cr), aluminum (Al), copper (Cu), gold (Au), silver (Ag), nickel (Ni), cobalt (Co), zirconium (Zr), iron (Fe), platinum (Pt) and zinc (Zn); alloys or compounds containing these metal elements (for example, nitrides such as TiN and silicides such as WSi2, MoSi2, TiSi2 and TaSi2); semiconductors such as silicon (Si); and electrically conductive metal oxides such as ITO (indium tin oxide), indium oxide and zinc oxide. The gate electrode can be made by forming a thin layer made of the above material on the insulating layer by a known thin film forming method such as a CVD method, a sputtering method, a vapor deposition method, an ion plating method, an electrolytic plating method, an electroless plating method, a screen printing method, a laser abrasion method or a sol-gel method. When the thin film is formed on the entire surface of the insulating layer, the thin film is patterned by a known patterning method to form the gate electrode in the form of a stripe. The opening portion may be formed in the gate electrode after the gate electrode in the form of a strip is formed, or the opening portion may be formed concurrently with the formation of the gate electrode in the form of a stripe. A patterned resist may be formed on the insulating layer in advance of the formation of the thin film, the gate electrode in the form of a stripe can be formed by a lift-off method. Further, vapor deposition may be carried out using a mask having openings conforming to the gate electrodes, or screen printing may be carried out with a screen having such openings. In these cases, no patterning is required after the formation of the thin film. Further, the gate electrode can be formed on the insulating layer by preparing an opening-portion-possessing stripe-shaped thin layer (foil) made of an electrically conductive material in advance and fixing such a thin layer on the insulating layer.
In the cold cathode field emission device having the first structure of a Spindt-type cold cathode field emission device, the material for constituting an electron-emitting portion can be made of at least one material selected from the group consisting of tungsten, a tungsten alloy, molybdenum, a molybdenum alloy, titanium, a titanium alloy, niobium, a niobium alloy, tantalum, a tantalum alloy, chromium, a chromium alloy and impurity-containing silicon (polysilicon or amorphous silicon).
In the cold cathode field emission device having the first structure of a crown-type cold cathode field emission device, the material for constituting an electron-emitting portion includes electrically conductive particles and a combination of electrically conductive particles with a binder. The material of the electrically conductive particles includes carbon-containing materials such as graphite; refractory metals such as tungsten (W), niobium (Nb), tantalum (Ta), titanium (Ti), molybdenum (Mo) and chromium (Cr); and transparent electrically conductive materials such as ITO (indium tin oxide). The binder includes glass such as water glass and general purpose resins. Examples of the general purpose resins include thermoplastic resins such as a vinyl chloride resin, a polyolefin resin, a polyamide resin, a cellulose ester resin and a fluorine resin, and thermosetting resins such as an epoxy resin, an acrylic resin and a polyester resin. For improving electron emission efficiency, preferably, the particle size of the electrically conductive particles is sufficiently small as compared with dimensions of the electron-emitting portion. Although not specially limited, the form of the electrically conductive particles is spherical, polyhedral, plate-like, acicular, columnar or amorphous. Preferably, the electrically conductive particles have such a form that exposed portions of the particles form acute projections. Electrically conductive particles having different dimensions and different forms may be used as a mixture.
In the cold cathode field emission device having the first structure of a plane-type cold cathode field emission device, preferably, the electron-emitting portion is made of a material having a smaller work function "PHgr" than a material for constituting a cathode electrode. The material for constituting an electron-emitting portion can be selected on the basis of the work function of a material for constituting a cathode electrode, a potential difference between the gate electrode and the cathode electrode, a required current density of emitted electrons, and the like. Typical examples of the material for constituting a cathode electrode of the cold cathode field emission device include tungsten ("PHgr"=4.55 eV), niobium ("PHgr"=4.02-4.87 eV), molybdenum ("PHgr"=4.53-4.95 eV), aluminum ("PHgr"=4.28 eV), copper ("PHgr"=4.6 eV), tantalum ("PHgr"=4.3 eV), chromium ("PHgr"=4.5 eV) and silicon ("PHgr"=4.9 eV). The material for constituting an electron-emitting portion preferably has a smaller work function "PHgr" than these materials, and the value of the work function thereof is preferably approximately 3 eV or smaller. Examples of such a material include carbon ("PHgr" less than 1 eV), cesium ("PHgr"=2.14 eV), LaB6 ("PHgr"=2.66-2.76 eV), BaO ("PHgr"=1.6-2.7 eV), SrO ("PHgr"=1.25-1.6 eV), Y2O3 ("PHgr"=2.0 eV), CaO ("PHgr"=1.6-1.86 eV), BaS ("PHgr"=2.05 eV), TiN ("PHgr"=2.92 eV) and ZrN ("PHgr"=2.92 eV). More preferably, the electron-emitting portion is made of a material having a work function "PHgr" of 2 eV or smaller. The material for constituting an electron-emitting portion is not necessarily required to have electric conductivity.
As a material for constituting an electron-emitting portion, particularly, carbon is preferred. More specifically, diamond is preferred, and above all, amorphous diamond is preferred. When the electron-emitting portion is made of amorphous diamond, an emitted-electron current density necessary for the flat-panel display can be obtained at an electric field intensity of 5xc3x97107 V/m or lower. Further, since amorphous diamond is an electric resister, emitted-electron currents obtained from the electron-emitting portions can be brought into uniform currents, and the fluctuation of brightness can be suppressed when such cold cathode field emission devices are incorporated into a flat-panel display. Further, since the amorphous diamond exhibits remarkably high durability against sputtering by ions of residual gas in the flat-panel display, cold cathode field emission devices having a longer lifetime can be attained.
Otherwise, the material for constituting an electron-emitting portion can be selected from materials having a secondary electron gain xcex4 greater than the secondary electron gain xcex4 of the electrically conductive material for constituting a cathode electrode. That is, the above material can be properly selected from metals such as silver (Ag), aluminum (Al), gold (Au), cobalt (Co), copper (Cu), molybdenum (Mo), niobium (Nb), nickel (Ni), platinum (Pt), tantalum (Ta), tungsten (W) and zirconium (Zr); semiconductors such as silicon (Si) and germanium (Ge); inorganic simple substances such as carbon and diamond; and compounds such as aluminum oxide (Al2O3), barium oxide (BaO), beryllium oxide (BeO), calcium oxide (CaO), magnesium oxide (MgO), tin oxide (SnO2), barium fluoride (BaF2) and calcium fluoride (CaF2). The material for constituting an electron-emitting portion is not necessarily required to have electric conductivity.
In the cold cathode field emission device having the second structure (flat-type or crater-type cold cathode field emission device) or the cold cathode field emission device having the third structure (edge-type cold cathode field emission device), the material for constituting a cathode electrode corresponding to the electron-emitting region can be selected from metals such as tungsten (W), tantalum (Ta), niobium (Nb), titanium (Ti), molybdenum (Mo), chromium (Cr), aluminum (Al), copper (Cu), gold (Au) and silver (Ag); alloys and compounds of these metals (for example, nitrides such as TiN and silicides such as WSi2, MoSi2, TiSi2 and TaSi2); semiconductors such as diamond; and a thin carbon film. Although not specially limited, the thickness of the above cathode electrode is approximately 0.05 to 0.5 xcexcm, preferably 0.1 to 0.3 xcexcm. The method for forming the cathode electrode includes deposition methods such as an electron beam deposition method and a hot filament deposition method, a sputtering method, a combination of a CVD method or an ion plating method with an etching method, a screen-printing method and a plating method. When a screen-printing method or a plating method is employed, the cathode electrodes in the form of stripes can be directly formed.
In the cold cathode field emission device having the second structure (flat-type cold cathode field emission device or crater-type cold cathode field emission device), the cold cathode field emission device having the third structure (edge-type cold cathode field emission device) or the cold cathode field emission device having the first structure of the plane-type cold cathode field emission device, the cathode electrode or the electron-emitting portion can be formed from an electrically conductive paste prepared by dispersing electrically conductive fine particles. Examples of the electrically conductive fine particles include a graphite powder; a graphite powder mixed with at least one of metal powders, a barium oxide powder and a strontium oxide powder; diamond particles or a diamond-like carbon powder containing nitrogen, phosphorus, boron and triazole; a carbon-nano-tube powder; an (Sr, Ba, Ca)CO3 powder; and a silicon carbide powder. It is particularly preferred to select a graphite powder as electrically conductive fine particles in view of a decrease in threshold electric field and an improvement in durability of the electron-emitting region. The electrically conductive fine particles may have the form of spheres or scales, or they may have any fixed or amorphous form. The particle diameter of the electrically conductive fine particles is not critical so long as it is equal to, or less than, the thickness or the pattern width of the cathode electrode or the electron-emitting portion. With a decrease in the above particle diameter, the number of electrons emitted per unit area can be increased. When the above particle diameter is too small, however, the cathode electrode or the electron-emitting portion may deteriorate in electric conductivity. The above particle diameter is therefore preferably in the range of from 0.01 to 4.0 xcexcm. Such electrically conductive fine particles are mixed with a glass component or other proper binder to prepare an electrically conductive paste, a desired pattern of the electrically conductive paste is formed by a screen-printing method and the pattern is calcined or sintered, whereby the cathode electrode which works as an electron-emitting region or the electron-emitting portion can be formed. Otherwise, the cathode electrode which works as an electron-emitting region or the electron-emitting portion can be formed by a combination with a spin coating method and an etching method.
In the cold cathode field emission device having the first structure of the Spindt-type cold cathode field emission device or the crown-type cold cathode field emission device, the material for constituting a cathode electrode can be selected from metals such as tungsten (W), niobium (Nb), tantalum (Ta), molybdenum (Mo), chromium (Cr), aluminum (Al) and copper (Cu); alloys and compounds of these metals (for example, nitrides such as TiN and silicides such as WSi2, MoSi2, TiSi2 and TaSi2); semiconductors such as silicon (Si); and ITO (indium-tin oxide). The method for forming the cathode electrode includes deposition methods such as an electron beam deposition method and a hot filament deposition method, a sputtering method, a combination of a CVD method or an ion plating method with an etching method, a screen-printing method and a plating method. When a screen-printing method or a plating method is employed, the cathode electrodes in the form of stripes can be directly formed.
The material for constituting an anode electrode corresponding to the electrode in the flat-panel display of the present invention can be properly selected depending upon the constitution of the flat-panel display. That is, when the flat-panel display is a transmission type (the display panel corresponds to a display screen), and when the anode electrode and the luminescence layer are stacked on the support member in this order, not only the support member but also the anode electrode itself is required to be transparent, and a transparent electrically conductive material such as ITO (indium-tin oxide) is used. When the flat-panel display is a reflection type (the back panel corresponds to a display screen), and even when the flat-panel display is a transmission type and the luminescence layer and the anode electrode are stacked on the support member in this order, the materials for constituting a cathode electrode and the gate electrode can be properly selected from the above materials in addition to ITO. The constitution of the anode electrode and the luminescence layer includes (1) a constitution in which the anode electrode is formed on the support member and the luminescence layer is formed on the anode electrode and (2) a constitution in which the luminescence layer is formed on the support member and the anode electrode is formed on the luminescence layer. In the above constitution (1), a so-called metal back layer electrically connected to the anode electrode may be formed on the luminescence layer. In the above constitution (2), a metal back layer may be formed on the anode electrode.
In the cold cathode field emission device having any one of the first to third structures, from the viewpoint of simplification of the structure of the flat-panel display, preferably, the projection image of the gate electrode in the form of a stripe and the projection image of the cathode electrode in the form of a stripe extend in directions in which these projection images cross each other at right angles. The electron-emitting region (constituted of one or a plurality of the cold cathode field emission device(s)) is formed in an overlapping region of the projection images of the cathode electrode in the form of a stripe and the gate electrode in the form of a stripe (the overlapping region corresponds to a region of one pixel in a monochromatic display or a region of one sub-pixel of three sub-pixels constituting one pixel in a color display). Such overlapping regions are arranged in the effective field of the back panel (which field works as an actual display screen portion) and generally arranged in the form of a two-dimensional matrix.
In the cold cathode field emission device having any one of the first to third structures, the plan form of the opening portion (form obtained by cutting the opening portion with an imaginary plane in parallel with the substrate surface) may be any form such as a circle, an oval, a rectangle, a polygon, a rounded rectangle or a rounded polygon. The opening portion can be formed, for example, by isotropic etching or by a combination of anisotropic etching and isotropic etching. As a material for constituting an insulating layer, SiO2, SiN, SiON and SOG (spin on glass) can be used alone or in combination. The insulating layer can be formed by a known method such as a CVD method, an application method, a sputtering method or a screen printing method. The insulating layer may be formed in the form of a rib. In this case, the insulating layer in the form of a rib can be formed in a region between neighboring cathode electrodes in the form of stripes or in a region between neighboring cathode electrode groups, each of which consists of a plurality of the cathode electrodes. The material for constituting an insulating layer in the form of a rib can be selected from known insulating materials, and for example, a material prepared by mixing a metal oxide such as alumina with a widely used low-melting glass can be used. The insulating layer in the form of a rib can be formed, for example, by a screen printing method, a sand blasting method, a dry filming method and a photosensitive method. The dry filming method refers to a method in which a photosensitive film is laminated on a substrate, the photosensitive film in a site where the insulating layer in the form of a rib is to be formed is removed by exposure and development and an insulating layer material is filled in an opening formed by the removal and is calcined. The photosensitive film is combusted and removed by the calcining, and the insulating layer material filled in the opening remains to constitute the insulating layer in the form of a rib. The photosensitive method refers to a method in which a photosensitive insulating material layer for a rib is formed on a substrate and the insulating material layer is patterned by exposure and development and then calcined.
In the cold cathode field emission device, a resistance layer may be formed between the cathode electrode and the electron-emitting portion. Otherwise, when the surface of the cathode electrode or the edge portion of the cathode electrode corresponds to the electron-emitting region, the cathode electrode may have a three-layered structure constituted of an electrically conductive material layer, a resistance layer and an electron-emitting layer corresponding to the electron-emitting region. The resistance layer can stabilize performances of the cold cathode field emission device and can attain uniform electron-emitting properties. The material for constituting a resistance layer includes carbon-containing materials such as silicon carbide (SiC); SiN; semiconductor materials such as amorphous silicon and the like; and refractory metal oxides such as ruthenium oxide (RuO2), tantalum oxide and tantalum nitride. The resistance layer can be formed by a sputtering method, a CVD method or a screen-printing method. The resistance value of the resistance layer is approximately 1xc3x97105 to 1xc3x97107 xcexa9, preferably several Mxcexa9.
In the flat-panel display of the present invention, the substrate for constituting the back panel or the support member for constituting the display panel may be any substrate or a support member so long as they have a surface made of an insulation material. The substrate or the support member includes a glass substrate, a glass substrate having an insulation layer formed on its surface, a quartz substrate, a quartz substrate having an insulation layer formed on its surface and semiconductor substrate having an insulation layer formed on its surface.
In the flat-panel display of the present invention, the back panel and the front panel can be bonded to each other with an adhesive or they can be bonded to each other with a combination of a frame made of an insulating rigid material such as glass or ceramic with an adhesive layer. When the frame and the adhesive layer are used in combination, a large distance between the back and display panels can be secured by selecting a proper height of the frame as compared with a case using an adhesive alone. While frit glass is generally used as an adhesive layer, a low-melting metal material having a melting point of 120 to 400xc2x0 C. may be used. The low-melting metal material includes In (indium, melting point 157xc2x0 C.); an indium-gold-containing low-melting alloy; tin (Sn)-containing high-temperature solders such as Sn80Ag20 (melting point 220-370xc2x0 C.) and Sn95Cu5 (melting point 227-370xc2x0 C.); lead (Pb)-containing high-temperature solders such as Pb97.5Ag2.5 (melting point 304xc2x0 C.), Pb94.5Ag5.5 (melting point 304-365xc2x0 C.) and Pb97.5Ag1.5Sn1.0 (melting point 309xc2x0 C.); zinc (Zn)-containing high-temperature solders such as Zn95Al5 (melting point 380xc2x0 C.); tin-lead-containing standard solders such as Sn2Pb98 (melting point 316-322xc2x0 C.); and soldering materials such as Au88Ga12 (melting point 381xc2x0 C.). All of the above subscript values show atomic %.
In the flat-panel display of the present invention, when the back panel, the display panel and the frame are bonded, these members may be bonded at the same time, or one of the panels and the frame may be bonded in advance at a first step and the other panel may be bonded to the frame at a second step. When these three members are bonded at the same time or the other panel is bonded to the frame at the second step in a vacuum atmosphere, the space surrounded by the back panel, the display panel and the frame comes to be a vacuum concurrently with the bonding. Otherwise, the space surrounded by the back panel, the display panel and the frame may be vacuumed to form a vacuum space after these three members are bonded. When the vacuuming is carried out after the bonding, the atmosphere for the bonding may have atmospheric pressure or reduced pressure, and the gas constituting the atmosphere may be ambient atmosphere or an inert gas containing nitrogen gas or a gas coming under the group 0 of the periodic low table (for example, Ar gas).
When the vacuuming is carried out after the bonding, the vacuuming can be carried out through a tip tube pre-connected to the back panel and/or the display panel. Typically, the tip tube is made of a glass tube and is bonded to a circumference of a through hole formed in a ineffective field of the back panel and/or the display panel with frit glass or the above low-melting metal material. After the space reaches a predetermined vacuum degree, the tip tube is sealed by thermal fusion. When the entire flat-panel display is once heated and then temperature-decreased before the sealing, properly, a residual gas can be released into the space, and the residual gas can be removed out of the space by the vacuuming.
As explained already, in the cold cathode field emission display, the accelerating voltage is required to be approximately 10 kilovolts or less, for example, approximately 6 kilovolts. In the above accelerating voltage, the electron energy loss peak is positioned near the outermost surface of the luminescence layer. Further, the electrons reach only as deep as approximately 0.2 to 0.3 xcexcm from the surface of the luminescence layer. When conventional phosphor particles were observed through a transmission electron microscope, it was found according to present inventor""s analysis that crystal defects such as dislocations, stacking faults and twin boundaries were present near the surface of the phosphor particle in a crystal defect density of 1xc3x97108 defects/cm2 or more.
Further, as already explained, in the luminescence layer, approximately 10% of energy of the energy beam (for example, electrons) contributes to light emission, and the remaining approximately 90% of the energy is converted to heat. That is, heat is generated near the surface of the luminescence layer to a great extent. As a result, when the luminescence layer is formed of phosphor particles, for example, made of a sulfide, sulfur as a constituent is dissociated in the form of a single atom, sulfur monoxide (SO) or sulfur dioxide (SO2), and the phosphor particles alter in composition or luminescence centers disappear. The above phenomenon comes to be more outstanding with an increase in the number of the crystal defects in the portion of the luminescence crystal particle which portion contributes to photon emission. Further, there is observed a phenomenon that the number of dislocations increases due to the heat generation of the luminescence layer. Further, with an increase in the number of the crystal defects, the luminescence efficiency of the luminescence crystal particles decreases.
In the present invention, the density of crystal defects such as dislocations, stacking faults and twin boundaries (to be simply referred to as xe2x80x9ccrystal defect densityxe2x80x9d hereinafter) in a region located from the surface of the luminescence crystal particle to a portion as deep as the energy beam reaches or penetrates or the electrons reach or penetrate (region of the luminescence crystal particle which region substantially contributes to photon emission) is 5xc3x97107 defects/cm2 or less, preferably 1xc3x97107 defects/cm2. As a result, not only the luminescence crystal particle can be improved in luminescence efficiency, but also the deterioration of the luminescence crystal particle can be prevented. When the initial value of brightness obtained by the photon emission of the luminescence crystal particles is assumed to be 100, the time period taken up to a brightness value of 50 at a current density of 0.2 xcexcA/cm2 in a cathode ray tube is 1xc3x97104 hours at a crystal defect density of 1xc3x97108 defects/cm2 In contrast, the above time period comes to be 1xc3x97106 hours at a crystal defect density of 1xc3x97107 defects/cm2. Further, when the current density in a cold cathode field emission display is 5 xcexcA/cm2, the above time period is 1xc3x97103 hours at a crystal defect density of 1xc3x97108 defects/cm2. In contrast, the above time period comes to 1xc3x97104 hours at a crystal defect density of 4xc3x97107 defects/cm2, and comes to be 1xc3x97105 hours at a crystal defect density of 1xc3x97107 defects/cm2.